The present invention relates generally to color cathode ray tubes and, more particularly, to a color cathode ray tube with an in-line type electron gun assembly for emission of a plurality of parallel electron beams extending in one plane.
Color cathode ray tubes, such as television picture tubes and display tubes, are widely employed as visual monitoring devices for use in receiving and displaying over-the-air broadcast TV programs or for use with a variety of types of information processing apparatus or equipment.
Color cathode ray tubes of this type are typically designed to include an evacuated outer envelope structure, which is structured from a panel portion having a fluorescent or phosphor screen formed on its inner surface, a neck portion accommodating therein an electron gun assembly for emission of more than one electron beam, and a cone-shaped portion, also known as a funnel section, for connecting the panel and the neck portion together. And, the electron gun assembly is typically designed to include in-line guns for giving off a plurality of parallel electron beams extending in one plane, i.e. the in-line plane.
FIG. 10 is a side view of an in-line electron gun assembly of the type used in prior art color cathode ray tubes, as seen from the in-line layout direction of electron beams. In FIG. 10, reference numeral xe2x80x9c20xe2x80x9d designates a cathode; 21 denotes a first grid electrode functioning as a control electrode; and, 22 indicates a second grid electrode acting as an acceleration electrode. The cathode 20, first grid electrode 21 and second grid electrode 22 constitute an electron beam generator unit (triode unit). Numeral 23 denotes a third grid electrode; 24 indicates a fourth grid electrode; 25 denotes a fifth grid electrode; 26 denotes sixth grid electrode; 27 denotes an anode; 28 denotes a shield cup; and 29 denotes a dielectric support structure (multi-foam glass).
Three electron beams as generated by the triode unit consisting of the cathode 20 and first grid electrode 21 and second grid electrode 22 are accelerated and pre-focused by an electron lens system, formed of the third grid electrode 23 and fourth grid electrode 24, as well as the fifth grid electrode 25. Then, the electron beams are focused by a main electron lens, formed of the sixth grid electrode 26 and anode 27, to direct the beams toward the phosphor screen. With the electron gun assembly of this type, the first grid electrode 21 and second grid electrode 22 and fourth grid electrode 24 are each comprised of a plate-shaped electrode, whereas those electrodes (fifth grid electrode 25, sixth grid electrode 26, and anode 27) making up the focusing electron lens and main electron lens are constituted from cup-shaped electrodes.
The third grid electrode 23 has an electron beam passage opening or hole on the side thereof facing the second grid electrode 22, which is less in aperture diameter than an electron beam passage hole on the side thereof facing the fourth grid electrode 24. FIGS. 11A and 11B are a front view and a partially broken sectional view, respectively, of the third grid electrode 23 of FIG. 10. FIG. 11A is a front view of the third grid electrode as viewed from the side facing the second grid electrode, whereas FIG. 11B is a partly broken sectional view from the side.
The third grid electrode 23 consists essentially of two separate electrode components. A first component 231 constituting the third grid electrode is a cup-shaped electrode component having electron beam passage holes 233 of small aperture or bore diameter. A second electrode component 232 making up the third grid electrode 23 is a plate-shaped electrode component having electron beam passage holes 234 greater in bore diameter than the electron beam passage holes 233. The third grid electrode is such that the first component 231 and second component 232 are bonded and soldered together to provide an integral or solid structure. Note that numeral 235 designates tabs to be embedded in the multi-foam glass for supporting the grid electrode.
FIG. 12 depicts a sectional view of the electrode structure as seen along line XIIxe2x80x94XII in FIG. 11A. The first component 231 is a cup-shaped component that has three electron beam passage holes 233 on the side facing the second grid electrode 22. The second component 232 is a plate-shaped component having three electron beam passage holes 234 greater in bore diameter than the electron beam passage holes 233. These two components are bonded and soldered together to thereby provide a third grid electrode 23 of solid structure. The aperture or bore diameter Db of the electron beam passage holes 233 on the second grid electrode side as formed in the first component 231 is less than the bore diameter Dt of the electron beam passage holes 234 on the fourth grid electrode side as formed in the second grid electrode 232 (Dd greater than Dt).
The third grid electrode 23 is thus arranged to employ two components that are soldered together with the center axes of electron beam passage holes of both components being identical to each other. However, the accurate positional alignment between the center axes of the electron beam passage holes of the two separate electrode components remains difficult, which in turn makes it difficult to assemble the third grid electrode with high accuracy. In addition, as respective components (the first component and second component) exhibit their own deviation in the manufacture thereof, the resulting third grid electrode 23 as manufactured by assembly of these components exhibits an even greater deviation. Furthermore, the third grid electrode is located adjacent to the triode unit. Due to such arrangement, minute deformation of the third grid electrode can significantly affect the electron beams that are being emitted.
The first component of the prior art third grid electrode shown in FIG. 12 is designed to have a rise-up portion 240, which is positioned outside of the one side electron beam passage hole in the in-line direction. In other words, the rise-up portion 240 is in close proximity to the side electron beam and yet far from the central electron beam. As a result, the electric field acting on the side electron beam will be different in shape from the electric field acting on the central electron beam. Such a difference between the electric field for the side electron beam and the electric field for the central electron beam in turn causes the central electron beam and the side electron beam to differ from each other in sectional shape also.
FIG. 13 is a process flow diagram for explanation of a manufacturing method of the prior art third grid electrode, which is formed of two components. The first component is formed by press-machining techniques into convex shape; then, three electron beam passage holes are formed in the top surface of the convex component. Barrel processing is then applied to the resultant structure to remove away burrs residing at the electron beam passage holes. The second component is manufactured by a method including the steps of applying press-machining to a plate body to form three electron beam passage holes, and then removing burrs at these electron beam passage holes through barrel processes.
Then, the first component and the second component are bonded with the centers of respective electron beam passage holes in alignment with each other, and the components are then soldered together into an integral or solid structure.
FIG. 14 is a side view of another example of a conventional in-line electron gun module, wherein those parts identified with the same reference numerals correspond to the same functional portions in FIG. 10. With an electron gun unit of this type, the first grid electrode 21 and second grid electrode 22, as well as the third grid electrode 23, are structured from plate-shaped electrodes, whereas those electrodes constituting the focus electron lens and main electron lens (i.e. fifth grid electrode 25, sixth grid electrode 26, and anode 27) are cup-shaped electrodes. The third grid electrode 23 consists of a single unitary plate body, which has three electron beam passage holes.
FIGS. 15A and 15B are a front view and a partly broken side view of the third grid electrode in FIG. 14, wherein FIG. 15A is a plan view as seen from the side facing the second grid electrode, whereas FIG. 15B is a partly broken side view. Press-machining is applied to the unitary electrode component to form therein electron beam passage holes 233, while simultaneously forming tabs 235 at two parallel opposite sides thereof in the in-line direction. A step-like height difference is present between the surface in which the electron beam passage holes 233 are formed and the surface in which the tabs 235 are formed. This step-like difference is provided for preventing the tabs 235 of the third grid electrode 2 and the tabs of the second grid electrode from approaching each other inside of the multi-foam glass support 29.
FIG. 16 is a sectional view of the structure taken along line XVIxe2x80x94XVI in FIG. 15A. Since the electron beam passage holes 233 of the plate-shaped third grid electrode 236 are formed by press-machining techniques, the second grid electrode side and the fourth grid electrode side have the same in bore diameter while the plate thickness (also called xe2x80x9celectrode lengthxe2x80x9d or alternatively xe2x80x9calong-the-tube-axis-direction lengthxe2x80x9d) thereof is at a limited value-typically 1 mm, more or less. Due to this, it is impossible to adequately control the diameter of an electron beam guided to reach the main lens, since the bore diameters of both electron beam passage holes (the hole on the entrance side of electron beam, and hole on the electron beam exit side) are equal.
Since the prior art electron gun units discussed above are designed in such a way that the third grid electrode is comprised of two separate electrode components, it is difficult to achieve the intended position alignment of electron beam passage hole center points between the first component and the second component. Unless such position alignment of the electron beam passage hole center points between the first component and the second component is suitably carried out, the resultant electron beams are likely to deviate in sectional shape.
In addition, it is also difficult to accurately dispose the first component and second component in a parallel fashion, which would easily result in occurrence of focusing degradation. The third grid electrode is a focusing electrode that is located near the triode unit and also is laid out at a location near a cross-over point. This in turn necessitates achievement of high-accuracy manufacturing when compared to the remaining electrodes involved. This is due to the fact that the influence of deformation of the third grid electrode upon the electron beams is more significant than that of the other electrodes upon the electron beams. To be brief, in case the electron beam diameter is minimal at the crossover point, if an electron beam changes in sectional shape at those points in close proximity to the crossover point, then its deformation will become greater at certain locations near or around the main lens at which the electron beam diameter becomes maximized.
Another problem is that the prior art third grid electrode suffers from the need for a significant number of pressing steps and of barrel processes, which in turn leads to increases in complexity of manufacturing procedures. On the other hand, the third grid electrode formed of the prior art unitary plate body is easily manufacturable. However, due to the form of such unitary plate, it is impossible, or at least greatly difficult, to increase the electrode length (length of the cathode ray tube in the direction of the tube axis) thereof. In addition, those electron guns which employ such unitary plate body have encountered a problem in that it is impossible to let electron beam passage holes on the side of neighboring electrodes (the second grid electrode side and fourth grid electrode side) be different in bore diameter from each other.
It is therefore an object of the present invention to avoid the problems inherent in the prior art and to provide an improved color cathode ray tube of high resolution having an electron gun assembly capable of simplified manufacture.
To attain the foregoing object, the electron gun of a cathode ray tube in accordance with the present invention is specifically arranged so that the third grid electrode consists essentially of a single unitary plate. The third grid electrode is provided with a cylindrical protuberant or bulged portions corresponding to respective ones of the cathode electrodes while forming more than one electron beam passage hole in the top surface of this bulged portion. A certain electron beam passage hole as formed on the side of the third electrode facing the fourth grid electrode may be designed to be greater than the electron beam passage hole on the second grid electrode side thereof.
Furthermore, in order to attain the above object, in accordance with the present invention, the prescribed bulged portion is formed to have a variable profile in such a way that its inner diameter gradually decreases.